The present invention relates to support units for optical sub-systems, in particular lens units, of microlithography systems. Furthermore, it relates to an optical projection system comprising such a support unit and an optical exposure apparatus comprising such an optical projection system. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image formed on a reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in several functionally distinct optical sub-systems. These distinct optical sub-systems may be formed by distinct lens units comprising a plurality of such optical elements, such as lenses and mirrors and other optical elements, of the optical system. Refractive lens units or at least mainly refractive lens units mostly have a straight axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, they generally have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the above optical sub-systems must be supported in a defined manner in order to maintain a predetermined spatial relationship between said optical sub-systems to provide a high quality exposure process.
In many cases, several distinct lens barrels are directly connected to each other to form a mechanical lens barrel unit as it is known, for example, from EP 1 168 028 A2 to Takahashi et al. In these cases, support to the lens barrel unit is provided via a support structure contacting a flange of one of the lens barrels. Often, different lens barrels are supported separately by one or more support structures. In either case the support structures supporting the lens barrels or lens barrel units typically are designed in the manner of an open skeleton framework. Such framework support structures are known, for example, from U.S. Pat. No. 5,638,223 and U.S. Pat. No. 6,529,264 B1 both to Ikeda as well as from U.S. Pat. No. 6,639,740 B1, U.S. Pat. No. 6,631,038 B1, U.S. Pat. No. 6,549,347 B1, U.S. Pat. No. 6,449,106 B1, U.S. Pat. No. 6,473,245 B1 all to Spinali.
Those open support structures may allow for an accurate positioning of the lens barrels with respect to each other. However, they show the disadvantage that, depending on the required spatial relationship between the lens barrels, the light path may have to pass open areas outside the lens barrels. For these areas a gas tight and light tight enclosure has to be provided for avoiding adverse effects on the quality of the exposure process. Such an additional enclosure adds to the overall complexity of the system
A further disadvantage of the above support structures lies within the fact that local deformations or position variations within single components of the support structure are likely to lead to position variations of the optical elements which largely affect the accuracy of the optical system and, thus, the quality of the exposure process. Furthermore, such variations cause extensive effort for re-adjusting the optical system.
Furthermore, from WO 03/012548 A1 to Kohl et al., it is known to support distinct optical sub-systems of a microlithography system via a support structure in the form of a housing-like frame structure composed of an upper frame structure element and a lower frame structure element. While the lower frame structure element supports a rather heavy elongated refractive optical sub-system in the form of an elongated lens barrel. the upper frame structure element only supports rather lightweight optical sub-systems in the form of axially short lens or lens and mirror groups.
This design may be useful for a microlithography system with only lightweight optical sub-systems located in the light path before any heavy elongated refractive optical sub-system. However, further heavy elongated refractive optical sub-systems located in the light path before said heavy elongated refractive optical sub-system mounted to the lower frame structure element seem to be mounted to open support structures as previously described. This again leads to the above disadvantages.
Furthermore, as it is known e.g. from U.S. Pat. No. 6,873,476 B2 to Shafer, configurations of optical elements may be used in microlithography systems where certain optical elements have to be arranged off-axis. U.S. Pat. No. 6,873,476 B2 to Shafer discloses a catadioptric microlithography projection unit with a folded optical axis. One of the mirrors used has to be laterally offset from the remaining optical elements resulting in a housing structure that deviates from the cylindrical design of the remaining part of the projection unit. This leads to a very complex design of the projection unit that has considerable disadvantages with respect to its thermal and dynamic properties. Furthermore, peripheral units, such as cooling devices etc., have also to be adapted to this complex design. Finally, assembly and adjustment of the projection unit are rather complicated since simple test methods available for rotationally symmetric units may not be used.